Divergent responses of particulate and mineral-associated organic carbon with soil depth under straw interlayer in saline-alkali soil

Burying straw deeply to establish interlayer is an effective strategy for saline soil improvement, and affects deep soil organic carbon (SOC) sequestration due to the landfill of exogenous carbon. However, the legacy effects of straw interlayer on deep SOC pools and its biotic regulatory mechanism remain unclear. Therefore, a field experiment was conducted with the buried interlayer at straw application rates of 0 Mg ha−1 (CK), 6 Mg ha−1 (SL6), 12 Mg ha−1 (SL12) and 18 Mg ha−1 (SL18). The straw interlayer was buried at 40 cm soil depth. Soil samples at the interlayer area (35–40 cm and 40–45 cm layers) and below the interlayer (45–55 cm, 55–65 cm and 65–75 cm layers) were collected to quantify changes in SOC fractions, as well as the mechanisms of soil microbial community and enzyme after a 6-year burying straw interlayer. Straw interlayer increased particulate organic carbon (POC), mineral-associated organic carbon (MAOC) and SOC contents, which were regulated by the straw application rate and soil depth. High amount of straw caused the positive effect of straw interlayer on SOC pools to extend deeper soil layer. Specifically, SL18 treatment with more straw inputs increased POC content by 33%-212%, MAOC content by 13%-28%, and SOC content by 22%-51% in relative to CK treatment, which was an effective practice to increase SOC sequestration in saline-alkali soil. Meanwhile, straw interlayer increased the POC/SOC, resulting in POC contribution to SOC greater than MAOC. Partial least squares path modeling showed that at the interlayer area, soil microbial community promoted the increases in POC and MAOC, contributing to SOC sequestration. Below the interlayer, soil enzymes promoted an increase in POC, contributing to SOC sequestration. These findings suggested that the regulation of SOC pool shifted from soil microorganisms to enzymes with soil depth, which will help us understand the regulatory mechanism of deep SOC pools after building straw interlayer in saline-alkali soil.